A conventional transpired fuel gas leak detecting device described in a patent document 1 is constituted to judge presence of a leak according to whether an internal pressure of a fuel tank, which is measured after leading in an outside air from the outside of the fuel tank and pressurizing the fuel tank for a predetermined time in a state in which a transpiration purge system is closed during operation of an internal combustion engine, has reached a set value.
In general, leak diagnosis is performed under a stable operation conditions such as idling time or low speed operation time so as not to be affected by a change in an operation condition of an internal combustion engine. Thus, it is necessary to set a measurement time long in order to secure detection accuracy. In this case, the number of times of leak diagnosis to be completed during operation is reduced excessively because of the change in the operation condition of the internal combustion engine. As a device for solving this problem, a transpired fuel gas leak detecting device described in a patent document 2 is constituted to judge presence of a leak by measuring an increase value of an internal pressure of a fuel tank due to self-transpiration of fuel after a predetermined time in a state in which a transpiration purge system is closed after an engine is stopped and, then, comparing the increase value with a value in judgment value tables of pressure increase values stored in advance having air volumes in the fuel tank calculated from a fuel level gauge and fuel temperatures calculated from a temperature sensor in the tank as parameters.
Patent document 1: JP-A-2002-195107 (paragraphs 0018 to 0020, FIG. 1)
Patent document 2: JP-A-2003-56416 (paragraphs 0003, 0004, and 0034 to 0042, FIG. 1)